It is conventional to apply sealant to the underside of container closure members in order to facilitate subsequent sealing attachment of the closure members to containers. Such sealant is normally applied in an annular pattern on the underside of each closure member in a manner such that, when the closure is attached to the container, the applied sealant will be located between the container rim and the closure member and, thus, seal the closure to the container.
One example of such a container closure is a can lid or "end", as it is often referred to in the can-making industry. During the manufacture of a can end, a sealant, such as a latex sealant, is conventionally applied to the underside of a curl region of the end. After the can is filled, the end is seamed onto the upper flange of the can and the previously applied sealant material facilitates sealing between the curl area of the end and the flange of the can to which it is attached in order to prevent leakage.
Another example of such a container closure is a bottle cap or "crown", as it is often referred to in the bottling industry. In a similar manner to the can end described above, bottle crowns are conventionally provided with a sealant material such that, when the crown is subsequently attached to a filled bottle, the sealant material will be located between the crown and the bottle, thus facilitating sealing attachment of the crown to the bottle.
To apply sealant to a container closure in a manner as described above, a sealant dispensing apparatus is generally used. Such an apparatus is often referred to in the industry, and may be referred to herein, as a "sealant dispensing gun" or simply a "gun". Such sealant dispensing guns typically include a supply line which supplies liquid sealant to the gun, and a valve, such as a needle valve, for allowing the liquid sealant to be selectively dispensed from the gun. A container closure is generally supported by a chuck member which locates the closure adjacent the gun in the desired position. The closure is then rotated at a high speed by the chuck while the sealant dispensing gun valve is opened, thus resulting in an arcuate, even application of liquid sealant onto the underside of the closure. After application, the liquid sealant cures to form a solidified ring of resilient sealing material.
The extent of the rotational coverage of sealant on the closure may be adjusted by controlling the valve "dwell time" which is a measure of the time that the valve remains in its open position. Rotational coverage of a closure member with sealant is dictated by the valve dwell time relative to the rotational speed of the chuck and attached closure member. The dispense rate of sealant through the valve may also be controlled by adjusting the extent to which the needle valve opens, also sometimes referred to herein as the "valve open limit" or simply the "open limit".
Sealant dispensing guns are conventionally found in either stationary, indexing machines or in rotary machines. In an indexing machine, a sealant dispensing gun is stationarily mounted while the container closures to be coated are indexed through the machine. An example of such an indexing sealant dispensing machine for applying sealant to bottle crowns is described in U.S. Pat. No. 3,412,971 of McDivitt for ELECTRICALLY-CONTROLLED VALVE APPARATUS AND CONTROL CIRCUIT SUITABLE FOR USE THEREIN, which is hereby incorporated by reference for all that is disclosed therein.
In a rotary machine, a plurality of sealant dispensing guns are generally mounted for rotation about an axis of rotation. A rotary can lid feed mechanism is provided having a series of pockets which locate a closure member beneath each of the rotating guns. Each of the closure members is then sequentially lifted, engaged by a chuck member and rotated while the adjacent sealant dispensing gun applies sealant thereto. Examples of rotary sealant dispensing machines are set forth in U.S. Pat. Nos. 4,262,629 of McConnellogue et al. for APPARATUS FOR APPLICATION OF SEALANT TO CAN LIDS; U.S. Pat. No. 4,840,138 of Stirbis for FLUID DISPENSING SYSTEM; U.S. Pat. No. 5,215,587 of McConnellogue et al. for SEALANT APPLICATOR FOR CAN LIDS and U.S. Pat. No. 5,749,969 of Kobak et al. for FLUID DISPENSING SYSTEM, which are hereby incorporated by reference for all that is disclosed therein.
Some sealant dispensing guns include valves which are operated by cams and mechanical linkage arrangements. In these types of machines, the valve dwell time and the valve open limit are generally dictated by the specific physical cam and cam follower arrangement used. Accordingly, adjusting the valve dwell time or valve open limit in such machines generally requires a time consuming and expensive process of replacing various mechanical elements. Examples of such mechanical actuation arrangements are illustrated in U.S. Pat. Nos. 4,262,629 and 4,840,138, referenced above.
More common in recent years, however, are sealant dispensing guns in which the sealant dispensing gun valve is actuated by an electrical solenoid device or devices. In such guns, the valve dwell time is dictated not by mechanical linkages and cams, but instead by the amount of time that the valve opening solenoid is energized. Accordingly, the use of such electrical solenoid devices allows the valve dwell time of a sealant dispensing gun to be easily varied. Examples of sealant dispensing guns utilizing electrical solenoid valve actuation devices are illustrated in U.S. Pat. Nos. 3,412,971, 5,215,587 and in 5,749,969, as previously referenced.
Since the cam actuation mechanism is eliminated in sealant dispensing guns having solenoid valve actuation devices, this type of gun generally also includes an adjustable mechanism for controlling the valve open limit. This adjustable mechanism may control the valve open limit by providing a movable stop for the valve stem or by moving the valve opening solenoid itself, or both.
In addition to solenoid valve actuation, some sealant dispensing guns also employ solenoid or motor actuated devices to adjust the valve open limit. Such guns allow remote control of the open limit and, thus, the rate at which sealant is dispensed from the gun when the valve is in its open position. Examples of sealant dispensing guns incorporating solenoid or motor actuated valve open limit devices are illustrated in U.S. Pat. Nos. 5,215,587 and 5,749,969 as previously referenced.
When applying sealant to a container closure member, it is desirable to closely control the number of rotations, or "turns" that the closure member makes while the sealant is being dispensed from the sealant dispensing gun. Since one turn equals 360 degrees of rotation, complete rotational coverage of the closure member with sealant always requires that sealant be applied for at least one turn. Some closure member manufacturers, however, desire that sealant be applied over multiple, e.g., two, turns in order to ensure comprehensive coverage. As previously described, the number of turns for which sealant is applied is determined by the time that the sealant gun valve dwells in its open position relative to the rotational speed of the chuck.
In solenoid actuated sealant dispensing guns, the valve dwell time is normally controlled by a timer which is initiated when the valve opening solenoid is first energized. After the desired amount of time passes, the timer causes the valve actuator solenoid to de-energize, thus causing the needle valve to move to its closed position. The time period set on the timer is dependent upon the rotational speed of the chuck and the number of turns of sealant coating desired.
This procedure is complicated by the fact that a solenoid "response time" exists between the time that the valve opening solenoid is first energized and the time that the valve actually moves to its open position. This response time represents the time required for the solenoid to energize to the extent necessary to cause movement of the valve member. Accordingly, in order to completely coat closure members with sealant over a specified number of turns, the time period set on the valve open timer must equal the desired valve open time plus the solenoid response time.
Further complicating this procedure is the fact that solenoid response times vary significantly over time. One cause of such response time variation is temperature fluctuation. As the ambient air in a production facility warms up during the day, for example, the valve actuator solenoid also warms, thus increasing the electrical resistance within the solenoid. As the solenoid operates, the electrical current supplied to the solenoid also creates heat in the solenoid and, thus, also contributes to increased electrical resistance in the solenoid. This increase in electrical resistance within the solenoid reduces the electrical current which flows through the solenoid and, thus, reduces the strength of the magnetic field produced by the solenoid. This reduction in magnetic field strength, in turn, reduces the response time of the solenoid.
Another factor impacting solenoid response time is the valve open limit setting. A larger valve open limit results in a larger solenoid gap when the valve is in its closed position. This larger gap increases the amount of magnetic force required to open the valve. Since additional time is required to build up this increased magnetic force within the solenoid, larger valve opening settings tend to induce longer valve actuation solenoid response times and smaller valve opening settings tend to induce shorter valve actuation solenoid response times. Valve opening settings are frequently adjusted during operation of sealant dispensing guns, thus inducing frequent variations in valve actuation solenoid response times.
Accordingly, in order to ensure that closure members are coated with a minimum number of turns, the time period set on the valve open timer must equal the desired valve open time plus the worst solenoid response time which is likely to be encountered during the day.
Many modern sealant application machines are capable of applying sealant to closure members at rates exceeding 2000 closure members per minute. In order to achieve such high-speed operation, it is necessary to rotate the rotary chucks and the attached closure members of the sealant application machines at extremely high speeds. In some machines, for example, the chucks may rotate at speeds of about 3500 rotations per minute.
In some applications, solenoid response time may vary, for example, from as fast as about 8 ms (0.008 seconds) to as slow as about 12 ms (0.012 seconds). Thus, the valve actuator solenoid response time may, at some times, be as much as about 4 ms (0.004 seconds) different than at other times, depending upon the operating conditions discussed above. In order to ensure that closure members are coated with sealant over a minimum number of turns, it is necessary to set the valve open timer for the desired valve open time plus the "worst case" valve response time. During periods when the solenoid response time is faster, however, this results in excess sealant material being deposited on the closure members. Considering the exemplary numbers set forth above, at times when the solenoid response time is at its fastest (i.e., about 8 ms (0.008 seconds)), for example, the needle valve will remain open for an extra 4 ms (0.004 seconds), during which time the chuck will rotate an extra 0.233 turns (with a chuck speed of 3500 rpm).
Accordingly, with the exemplary numbers set forth above, up to nearly a quarter turn of extra sealant material may be applied to each closure member at certain times during the operation. This excess sealant represents significant cost in wasted sealant material. Such "overturning" also can impair the ability of the closure member to be effectively sealed to a container since a portion (nearly a quarter turn in the above example) of the closure member has a thicker deposit of sealant material than the remainder of the closure member.
As previously described, the valve open limit dictates the dispense rate of sealant from a sealant dispensing gun. Thus, if the valve open limit is too large, an excessive amount of sealant may be applied to the closure members, resulting in wasted sealant material. If, on the other hand, the valve open limit is too small, then too little sealant may be applied to the closure members. This condition may impair the ability of the closure members to be properly sealed to their respective filled containers.
It has been found that, even if the valve open limit is initially properly set, the dispense rate from a sealant dispensing gun may change over time. Such fluctuations in dispense rate are believed to be caused by various factors. One such factor is the temperature of the sealant material being dispensed from the gun. The viscosity of the material tends to decrease as temperature increases and increase as temperature decreases. Since sealant material having a higher viscosity tends to flow more easily than sealant material having a relatively higher viscosity, such temperature fluctuations tend to cause corresponding fluctuations in the dispense rate of sealant material.
Another factor which may impact dispense rate is wear which commonly occurs in the nozzle portions of the dispensing guns. As sealant is dispensed over time, the nozzle openings tend to become enlarged, causing an increase in sealant dispense rate.
Sealant dispense rate may also be impacted by minor fluctuations in the pressure of the sealant material. As can be appreciated, an increase in this pressure will generally cause an increase in the dispense rate while a decrease in pressure will cause a decrease in the dispense rate.
For the reasons set forth above, it is desirable to monitor the flow rate of sealant dispensing guns over time and to adjust the valve open limit as necessary to maintain the proper flow rate. One conventional method of monitoring flow rate is a sampling technique in which closure members are weighed after application of sealant by a dispensing gun. Generally, the closure members must be heated in an oven in order to cure the sealant material prior to making this weight measurement. The difference between the measured weight and the known weight of the closure member without the sealant is indicative of the weight of sealant applied to the closure member. Based upon the weight of sealant applied, the sealant gun valve open limit may be adjusted as necessary. Closure members may be selected for weight sampling at predetermined time intervals in order to track and correct for variations in dispense rate.
The sampling method described above represents a less than ideal flow rate monitoring method for several reasons. For example, the sampling method is labor intensive. In addition, there is a lag time between when the sampled closure members are coated with sealant and when the weight measurement is actually made. Due to the high speed of most sealant dispensing gun machines, many defective closure members may be produced during this lag time.
In another known method of monitoring flow rate, a flow measuring device is provided in-line with the conduit which supplies sealant to a sealant dispensing gun. Such a flow measuring device may generally comprise a pair of pressure transducers separated by a restrictive orifice. The difference in pressure read by the two transducers is indicative of the flow rate through the orifice and conduit and is, thus, also indicative of the flow rate of the dispensing gun when the valve is opened.
In this manner, such a flow measuring device is able to monitor the rate of flow and the duration of flow and, thus, the volume and weight of flow for each valve open cycle may be calculated and displayed on an operator information panel. The machine operator may then track the weights and make adjustments to the dispensing gun valve open limit as necessary. An example of a system employing a flow measuring device as described above is disclosed in an advertising publication entitled "PMC Pressure Monitoring Control System for Reciprocating Compounding Systems" and distributed by Tech-S, 12755 Merriman Rd., Livonia, Mich. 48150, which is hereby incorporated by reference for all that is disclosed therein.
Although the flow measuring device described above reduces lag time, it still requires human operator intervention. Even when used in conjunction with sealant dispensing guns incorporating solenoid or motor actuated valve open limit devices, the data from the flow measuring device must first be handled by a human operator who must then manually make the valve open limit adjustment.
Thus, it would be generally desirable to provide an apparatus and method which overcomes these problems associated with solenoid valve response time and sealant dispense flow rate as described above.